END MILL

Abstract: An end mill for cutting machining of metals has two or more main cutting edges (5'). The partitions between the main cutting edges circumferentially around the rotation axis of the end mill are different. The maximum angle value of a partition angle is defined by the formula: omega n = (360 +/- sigma )/z where omega n is the partition angle, sigma is a constant which shall not exceed 90 and z is the number of main cutting edges. However, the smallest difference between at least two partition angles shall not be less than 4 DEG /z. By this differentiated partition, vibrations are avoided to a high degree, which otherwise sometimes occur at known end mills.

Description: END MILL The present invention relates to an end mill comprising the features as defined in the characterizing portion of claim 1. End mills according to the invention pertain to an advanced group of tools used in difficult operations, wherein the requirements on surface smoothness are very high. This is the case for instance in the aircraft and space industries, where no unevenness or kerfs may be present which could lead to breakage and even to a crash. Furthermore, in aircraft industry wall dimensions have been gradually reduced over the years in order to decrease the weight of different components, and this of course may aggravate the tendency to vibrations. For the time being, there is a trend towards smaller and, therefore, also frailer milling machines, with higher and higher spindle rotation speeds (15 000 to 60 000 rev/minute) . Also this trend increases the occurrence of vibration problems. From these factors, one can conclude that there are two additive sources of vibration in the aircraft industry: thin-walled materials and frail high speed machines. These vibration problems are further stressed by magnet and/or air cradled machine spindles. End mills are normally used in such machines. A first object of the present invention is thus to provide an end mill which reduces, or even eliminates vibration problems. A further object of the present invention is to generate smooth surfaces on thin-walled work-pieces. These and further objects have been attained in a surprisingly simple manner by constructing an end CONFIRMATION mill comprising the features defined in the characterizing portion of claim 1. Thus, with an end mill according to the present invention, Revalues as low as 0,4 μm have been achieved on thin-walled work- pieces by so called HSM-machining. For illustrative but non-limiting purposes, the invention will now be described in more detail with reference to the appended drawings which show some preferred embodiments of the invention, and wherein like members bear like reference numerals: Figure 1 shows an end mill with two cutting edges in a side view according to prior art; Figure 2 shows a ball end mill with two cutting edges in a side view according to prior art; Figure 3 shows an end mill with three cutting edges according to the invention in a side view; Figure 4 shows the cross-section A-A in Fig. 3; Figure 5 shows the corresponding cross-section of an end mill with two cutting edges; Figure 6 shows a perspective view of an end mill according to the invention; and Figure 7 is a side view of a further end mill according to the invention. Both the end mills according -to the prior art (Fig. 1 and 2) and those according to the present invention (Fig. 3 to 7) , consist generally of an elongated cylindrical body 1. It can be produced of several different metallic materials, such as high speed steel, cemented carbide or a cermet. It can also be made of a material known as "Coronite" (registered trademark of Sandvik AB) which consists of 30 to 70 % by volume of submicronic hard substances in a metallic bonding phase, cf. the Swedish patent SE-C-392 482. This material has a superior wear resistance, comparable with advanced HSS, and can therefore be placed in the property-related gap between cemented carbide and HSS. Further, in the Swedish patent SE-C- 440 753 it has been demonstrated how superior compound tools have been obtained with inter alia the above material in the areas being exposed to high cutting speeds, and with HSS in the center, for drilling applications. This compound material can advantageously also be used at the manufacture of end mills according to the present invention, in particular in end mills that shall also be capable of boring. According to prior art (Fig. 1 and 2) , a number of helically twisted lands 2, normally two, three or four, are arranged at constant intervals in the circumferential direction around the central axis of the tool . Lands 2 are delimited by flutes 3. Since the lands 2 on one and the same end mill have the same radial extension, and the same geometrical form in general, and since the distance between two adjacent lands always is the same, it follows that all the flutes 3 also have the same geometrical form. It can further be concluded that all main cutting edges 4 give rise to the same pace of equal cutting force pulses, which may cause vibrations and resonance, in particular when, the working pace corresponds to the self frequency of the tool . Contrary to the prior art discussed above, the end mill according to the present invention has a differentiated circumferential partition of the main cutting edges around the rotation axis of the tool, in order to minimize or even eliminate vibration problems. In other words, the circumferential angle between at least two cutting edges is different. This differentiated partition gives rise to differently large cutting force pulses for each individual cutting edge, and differently large time intervals between the cutting force pulses. The construction is illustrated in Fig. 4 and 5, in which the main cutting edges are designated by 5 and 5' , respectively. After several tests, it has been concluded that a suitable maximum value of a differentiated partition angle can be defined by the formula Cj n = (360 ± σ)/z where C n is the partition angle, σ is a constant and z is the number of main cutting edges. Generally, the value of σ should not exceed 90°. Suitably, it should not exceed 60° and preferrably it should not exceed 40°, and even more preferrably, 35°. Most preferrably, it should not exceed 30°. At the other end, the difference between at least two partition angles should generally not be smaller than 4°/z , and preferrably not smaller than 8°/z. In the end mill with three main cutting edges according to Fig. 4, the partition angles ώ/ n1 , CJ nZ and n3 are about 128, 119 and 113°, respectively. According to Fig. 5, the end mill has two main cutting edges with a differentiated partition where 0> nl is 160° and ty- 2 is 200°. Differentiated end mills with four cutting edges can also be produced, although they do not represent a preferred embodiment. However, for all differentiated end mills according to the invention, the differentiation should not be less than 1 degree between at least two partitions, along the whole longitudinal extension of the tool. End mills with three or four main cutting edges should have at least two differentiated partitions angles. Thanks to the differentiated partition, surprisingly good cutting data have been obtained. In comparison with corresponding end mills with an equal partition, one has obtained larger feeds, higher cutting speed, larger cutting depth and smoother surfaces (R a -value of about 0,4 μm) . Due to the decreased vibration tendency it has also been possible to use longer tools. The pitch angle W can vary within wide ranges, as long as the pitch angle is substantially the same for all main cutting edges. The fact that substantially the same pitch angle is provided for all main cutting edges of the end mill has brought about several beneficial effects. For instance, the manufacture of the end mill is considerably simplified. Further, it is easier to optimize a common pitch angle for a certain type of material and quality of the work-piece. Moreover, different pitch angles could possibly extinguish the differentiated repartition at a certain level along the axis of the tool. Generally, the pitch angle is between 5 and 60°, preferrably between 25 and 50° and in particular between 35 and 45°. In particular at so called package milling, the pitch angle is small, about 6° . The cutting geometry may be negative; however, in order to keep the cutting forces down, a positive rake angle γ is preferred, see Fig. 5. Generally, a rake angle of between 0 and 20° is chosen, preferrably between 5 and 15°, and in particular between 8 and 12°. Also the rake angles do not have to be equal for the different main cutting edges of one end mill. In order to guarantee a sufficient free play in relation to the work piece, not too large a portion of the relief side 6 behind the cutting edge in the direction of cutting, may be in contact with the work piece. A suitable clearance angle α is between 5 and 20°, partially depending on the cutting diameter of the end mill. Normally, an end mill with a small diameter has a larger clearance angle than one with a larger diameter. Thus, an end mill with a cutting diameter of 25 mm can have a clearance angle of about 11°, while an end mill with a cutting diameter of 4 mm can have a clearance angle of about 17°. Further, the relief angle α can also be differently large for the different main cutting edges. The cutting diameter of the tools is normally between 4 and 40 mm. The operative end of the end mill can have different shapes. It can have a substantially straight end, in accordance with the prior art in Fig. 1, or a round shape according to the prior art in Fig. 2, whereby the latter usually is called a ball end mill and the radius of the round end surface usually corresponds to the cutting radius of the end mill . The advantage of this embodiment is that the end mill also can drill. The end mills with a straight end can also be conferred a drilling capability by providing one or several end cutting edges 7, 8 on the operative end surface of the end mill. In order to make possible an effective boring function, at least one of these end cutting edges shall reach the rotation axis of the tool. According to Fig. 6, end cutting edge 7 reaches the rotation axis, while end cutting edge 8 is interrupted by a recess 9, which inter alia facilitates chip conveyance. The edge corner between the end cutting edge and the main cutting edge can be sharp as in Fig. 6 or be chamfered with an inclined face, as in Fig. 7, or have a radius. In order to produce narrower chips, the main cutting edges can be provided with chip dividers 10, in accordance with Fig. 7. The illustrated embodiments relate to solid end mills, i.e., the cutting edges are parts of the same piece as the tool per se, or the cutting edges have been brazed upon the tool. However, within the frame-work of the present invention, it is also possible to form the tool as an end mill with indexable cutting inserts. According to this embodiment, the main cutting edges, and possibly also the end cutting edges, are formed by one or more cutting inserts which have been screwed upon the tool. Of course-, in this embodiment the differentiated partition according to the present invention is included. For end mills with indexable cutting inserts also other materials can be used for the manufacture of the carrying tool, such as steel and aluminum. In order to make the end mill tools maintain a good dimension accuracy also at long tool lives and demanding cutting data, they can be covered with a thin layer of TiN, Ti(C,N) and/or Ti(Al)N. Preferrably, TiCN is used in a thickness of 2 to 4 μm, particularly in view of the excellent adhesion obtained with this material. The coating of the operative parts of the end mill with a thin layer is suitably effected by the well known PVD-process (PVD stands for Physical Vapour Deposition) .

Claims: C L A I M S 1. End mill for cutting machining comprising a generally elongated cylindrically shaped body with two or more main cutting edges (5, 5') with flutes (3) therebetween, whereby the partition in the circumferential direction between these main cutting edges is differentiated, c h a r a c t e r i z e d in that the maximum value for a partition angle is defined by the formula CO n = (360 ± σ)/z where C-J n is the partition angle, σ is a constant that shall not exceed 90° and z is the number of main cutting edges, and in that the smallest angle difference between at least two partitions is 4°/z. 2. End mill according to claim 1, c h a r a c t e r i z e d in that σ shall not exceed 60° . 3. End mill according to claim 1, c h a r a c t e r i z e d in that σ shall not exceed 40°. 4. End mill according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the smallest angle difference between at least two partitions is 8°/z. 5. End mill according to any of the previous claims, c h a r a c t e r i z e d in that in its operative portion, the end mill comprises two, three or four main cutting edges (5, 5') with flutes (3) therebetween. 6. End mill according to any of the previous claims, c h a r a c t e r i z e d in that its operative end surface is provided with end cutting edges (7, 8) , in order to make possible drilling. 7. End mill according to any of the previous claims, c h a r a c t e r i z e d in that its operative end surface is substantially straight. 8. End mill according to any of the previous claims, c h a r a c t e r i z e d in that its operative end surface is rounded.

Inventors: LUNDBLAD MIKAEL, LUNDBLAD, MIKAEL
Applicants: SANDVIK AB
Application year: 1994
Main classification: B23C5
IPC codes: B23C5